Intrinsic intensity fluctuations in random lasers

In 1968 Letokhov wrote his pioneering paper 1 in which he predicted that light amplification through stimulated emission is possible in a random medium with gain. A preeminent experimental demonstration of such a random laser was published by Lawandy et al. in 1994 2 . Typical phenomena of the random laser are a low threshold in the power conversion, spectral narrowing, and sharp features “spikes” in the emitted spectrum for both picosecond 3–6 and nanosecond 7 pump pulses. Due to their low threshold and their ease of production, random lasers are expected to be used in many applications, such as coding of clothing 8 and detection of dangerous materials 9 . The theoretical search for the underlying principles of random lasers intensifies, focusing on their statistical properties and fluctuations 10–13 . Recently, a new random-laser phenomenon was described by Anglos et al. 14 . They observed shot-to-shot intensity fluctuations in the emitted light, which were not caused by fluctuations in the pump source. In their experiments, the intensity fluctuations occur for nanosecond pump pulses, but not for picosecond pump pulses. The physical understanding of the underlying principle of these fluctuations is relevant for applications and our perception of random lasers. In this paper we present a quantitative experimental and theoretical study of the statistics of these shot-to-shot intensity fluctuations of random laser systems that have different realizations of disorder for every pump pulse, and develop a model that clarifies the existence of these fluctuations. Our aim is to provide physical understanding of the fluctuations. With such a model, experimental conditions can be tailored to control the fluctuations. This paper is organized as follows. In Sec. II the experimental setup is described, followed in Sec. III by the experimental observations. A model based on the number of laser modes in the random laser is presented in Sec. IV. We compare our model with experimental observations in Sec. V.